A number of non-invasive methods of examining internal bodily organs, or sections of such organs, have become popular for diagnosing a variety of illnesses. One of these-techniques is called Positron Emission Tomography (PET) or Positron Emission Transaxial Tomography (PETT). In this method of developing internal bodily images, an array of sensors detects gamma rays emitted from tissues after the subject has been administered a natural biochemical substance (for example, gases, glucose or fatty acids) into which positron-emitting radio-isotopes have been incorporated. A computer calculates the paths of the gamma rays (which result from collisions of positrons and electrons) and interprets the data to generate a tomographic image. The resultant tomogram represents local concentrations of the isotope-containing substance in the tissues. By proper choice of isotope-containing substances, various processes such as brain function, local blood flow, blood volume and other metabolic processes can be studied.
The short-lived radio-isotopes are administered by intravenous injection or by having the subject inhale a gas containing small quantities of the radio-isotope. Isotopes which are often incorporated into such gases or injections are carbon-11, nitrogen-13, oxygen-15 and fluorine-18. In present PET facilities, these radioisotopes are derived from boron, oxygen or carbon, nitrogen, and oxygen or neon targets, respectively, by bombarding the targets with high-energy protons or deuterons obtained from a particle accelerator.
In recent years, the need for radioisotope sources has been addressed in two ways: through the development of lower cost, automated accelerator systems with multiple radioisotope and pharmaceutical production capabilities, and through the development of specialized single isotope, single pharmaceutical delivery systems. In cyclotron-based systems, nitrogen-13 is typically produced via the .sup.16 O(p,.alpha.).sup.13 N reaction in liquid H.sub.2.sup.16 O or via this reaction in combination with the .sup.13 C(p,n).sup.13 N reaction in a water and charcoal slurry.
Producing radioisotopes for PET is further described in U.S. Pat. No. 4,812,775, granted on Mar. 14, 1989 and entitled ELECTROSTATIC ION ACCELERATOR, herein incorporated by reference.
Ruben et al. have reported the extraction of nitrogen-13 from irradiated graphite for biological studies as early as 1940. Varner and co-workers have bombarded thin graphite targets with deuterons and extracted the nitrogen-13 in acid. Clark and Buckingham report the extraction of .sup.13 N as .sup.13 NN via oxidation with carbon dioxide sweep gas, while Darquennes et al. have shown that .sup.13 NN can be extracted using nitrogen as a sweep gas at a temperature of 1880.degree. C. Both the approach of Clark and Buckingham and that of Darquennes et al. produce primarily .sup.13 NN.
Rubidium-82, another agent that can be used to measure myocardial blood flow, is generator produced. The column containing the parent radionuclide, .sup.82 Sr, is quite expensive and generally must be replaced on a monthly basis. A computer controlled infusion system is also needed to administer the nuclide.